The present invention relates to cell phone charging circuit of a USB interface and, more particularly, to a cell phone charging circuit which uses a power source from a USB interface and has the functions of limiting current, compensating temperature and automatically adjusting voltage.
The universal serial bus (USB) has become an inevitable interface in personal computer, including desktop computer, notebook computer, flat-panel computer, and palm computer. Even the Macintosh computer system adapts the USB as the standard interface. The USB provides a tree-like connection to various types of external devices. It does not only provide high-speed data transmission, but also provides a 5V DC power source to the external devices when the computer is either on or off.
Due to the continuous enhancement of performance, the portable computer (including notebook computer, flat-panel computer or palm computer) has gradually dominated over the desktop computer in the market. Meanwhile, cell phones also become so common that each person possess one or more than one cell phones. The portable computer can log on intenet via the cell phone. The space limitation is relieved, and the user can surf the internet everywhere. However, as the cell phones have a very small dimension; and consequently, the battery power is very limited. If the cell phones can be charged through the USB interface, the application can be broadened and become more popular.
The typical cell phone battery includes Ni_MH, L-ion or Li-polymer batteries. The voltage rating for these batteries is 3.6V. The power supplied by a USB interface is normally 5V. Therefore, a voltage drop is required for applying the USB interface to the cell phone. There are three conventional types of cell phone chargers. The first is the pulse type, which uses software or hardware to convert the output voltage and current into high-frequency pulses, such that the battery is intermittently charged. This is the best type charging method. The performance and lifetime of the battery are optimized by this method. However, the cost of the pulse type is very high. The second type is the linear type (continuous type). That is, a DC current with limited voltage and current is used for charging the battery, such that the battery is protected from being over charged. As the voltage of the battery is increased, the output voltage increases automatically. The performance is superior to the first type, however, this type is more economic. The third type is the simple type, which uses a diode for voltage drop and rectification. Some charger even removes the diode, such that the voltage limiting, current limiting and temperature compensating functions are not provided. This type of charger shortens the lifetime of the battery.
The present invention provides a cell phone charger of a USB interface, which uses the simplest circuit devices and structure to achieve voltage limiting, current limiting, temperature compensating and automatic voltage adjusting functions of a linear charger.
The cell phone charger includes a PNP transistor, two diodes with negative temperature coefficient, a current limiting resistor, a biased resistor and a filter capacitor. The emitter of the transistor is serially connected to the current limiting resistor, such that two parallel paths are formed by the current limiting resistor and the emitter and base of the transistor and the diodes. Thereby, the voltage between the emitter and the base of the transistor is the same as the voltage of one of the diode; and consequently, the voltage of the current limiting resistor is the same as the voltage of the diode. The current limiting function is thus implemented. When the temperature of the battery increases by charging, the negative thermal coefficient of the diodes reduces the voltage thereof. Therefore, the output current is decreased to provide the temperature compensation. When the output current of the battery decreases as the voltage of the battery increases, the output voltage is increased as the voltage between the emitter and collector of the transistor drops. The automatic voltage adjustment is achieved.